Sample substrate surfaces such as wafer surfaces are important in determining the quality of the microelectronic fabrication product. Semiconductor substrate crystals, also called boules, are mechanically trimmed to a proper diameter forming a wafer. The wafer is dipped in a chemical etchant to even out any mechanical damage. The wafer also undergoes a series of cleaning steps. These steps remove residual contaminants on the wafer surface. They are of particular interest because the overall device quality may be negatively affected by the presence of trace contaminants.
Epitaxial growth applications demand contaminant-free sample substrate surfaces. A polished bulk grown sample substrate such as a wafer is used as a foundation. A much higher-grade epitaxial layer is grown onto the wafer surface. The new crystal layer has a similar crystal structure to the base wafer. The improved grade is compatible for direct device fabrication. Achieving a chemically pure substrate surface for epitaxial growth is critical to obtaining a high yield from the subsequent fabrication of devices from the epitaxial material.
To ensure a high quality yield, contaminants must be removed from the substrate surface without damaging or undesirably altering the surface. Total metallic impurities on the cleaned surface should be less than 10.sup.10 atoms/cm.sup.2 ; particles greater than 0.1 micrometer in size should be present at less than 0.03 particles /cm.sup.2. The desired cleaning protocol should also utilize contamination-free and non-corrosive cleaning agents that are safe and economical for use in batch manufacturing processes.
One major cleaning approach known in the art is wet cleaning. Several types of chemical contaminants, including molecular, ionic, atomic, and gaseous contaminants, can be found on the substrate surface. Wet cleaning techniques use organic solvents for pre-cleaning, the acids within ultrasonic baths to etch away defects, followed by a deionized water rinse, and finally drying. Several effective cleaning agents are described in the literature. Some of these include Hydrofloric acid solutions (e.g. 49 wt % HF), Sulfuric acid--Hydrogen peroxide mixtures (e.g. 98% H.sub.2 SO.sub.4, 30% H.sub.2 O.sub.2), and NH.sub.4 OH, HCl solutions. The acidic and oxidizing solutions selectively form soluble alkali and metal salts by dissolution and or complexing the contaminants. After this reaction, contaminants can then be rinsed away. The surface is dried to remove any residual matter.
Wafer drying is an important step for ensuring that a cleaning process is successful in eliminating contamination. Within the semiconductor industry, a variety of drying techniques have been developed with varying degrees of success. Three such basic methods include: spin drying, Marangoni surface tension drying, and single solvent vapor drying.
Spin drying technique is believed to have some disadvantages. Recontamination can occur from residual impurities on the back side of the sample. In addition, the vacuum apparatus used to secure the wafer while spinning can cause stress-related damage to softer semiconductor materials such as Indium phosphide (InP), Indium arsenide (InAs), or Indium antimonide (InSb).
Another technique, known as Marangoni drying, relies on the change in water/air surface tension as a sample substrate is slowly removed from an alcohol/water solution. The removal speed that results in a dry surface is strongly related to the surface roughness. Hence, for uniform surfaces, such as planar semiconductor substrates, Marangoni drying is a suitable technique. However, for materials with patterned surfaces, Maragoni drying is less effective since it does not provide a means for properly drying all the nonplanar and transition regions.
A third technique, single solvent vapor drying primarily using isopropyl alcohol (IPA), is more effective than spin drying and has recently experienced a great amount of development. Several patents in this area, including U.S. Pat. No. 5,454,390 by Lawson et. al, U.S. Pat. No. 5,371,950 and 5,054,210 by Schumacher, and U.S. Pat. No. 4,777,970 and U.S. Pat. No. 4,736,758 by Kusuhara basically use a two zone process to dry wafers. The first zone consists of an ambient saturated with vapors from a solvent such as isopropyl alcohol (IPA). A sample substrate just removed from a deionized (DI) water rinse cycle is left in the first zone long enough to allow the IPA to displace the DI water from the surface. After a sufficient amount of time, the sample substrate is moved into the second cooler zone. The inventors claim that in the cooler second zone, rapid condensation and gravitational removal of IPA from the surface occurs.
However, during an investigation into the use of single vapor drying technique, the present inventor found that the technique does not sufficiently remove the contaminants. Contaminants present on the substrate surface are revealed during subsequent epitaxial growth. During attempts at epitaxial regrowth on gratings for first order distribute feedback (DFB) lasers, with features as small as 400 angstroms, morphology problems emerged. These morphology problems had a very negative impact on the number of devices fabricated from the material. This low yield is undesirable. The contamination is most likely caused by the presence of droplets of solvent and residue that remain on the surface even after the IPA vapor dry, flash cooling, and high purity N.sub.2 blow drying.
In light of the deficiencies encountered in these wafer drying techniques, the final drying step still represents a performance bottleneck in manufacturing lines developed to fabricate deep-sub micron devices. Trace amounts of water vapor and other contamination that remain on the surface of patterned substrates after a thorough chemical cleaning should be removed. Thus, it is desirable to provide a vapor drying system that removes both moisture and cleaning agents from the surface of planar and non-planar sample substrates.